Not applicable.
The present invention relates generally to a device, capable of operating at high pressure, for the detection of fluid flow rates over a wide range of flow rates ( less than 1 nL/min to  greater than 10 xcexcL/min) and particularly for measuring fluid flow rates in microfluidic devices.
Numerous types of mass flow meters are generally available. Some are based on viscous drag, others employ the Venturi effect to measure a pressure drop, still others use positive displacement either rotary or linear. For low flow rates, local heating of a passing fluid and a variation in the resistance of a resistive element or the current/voltage required to maintain a constant temperature/current in the resistive element is conventionally used to determine fluid flow rate, wherein the fluid can be either a liquid or a gas. Rudent et al. in U.S. Pat. No. 6,354,150 xe2x80x9cSensor for a Capillary Tube of a Mass Flow Meterxe2x80x9d, issued Mar. 12, 2002, describe a mass flow rate sensor based on the time-of-flight of a thermal pulse. The time-of-flight of a small volume of liquid is measured from an upstream position where the temperature is modified to a downstream detector. However, the thermal sensor requires specially insulated components to avoid thermal transients, which can be a disadvantage.
The invention is directed, in part, to a device and method for measuring very small fluid flow rates (nL/min range), and particularly for measuring flow rates in microfluidic devices. The invention operates by producing localized compositional variations in the fluid at two distinct locations along the flow axis. The compositional variation, or pulse, that is subsequently detected downstream from its point of creation is used to derive a flow rate. The pulse, comprising a narrow zone in the fluid whose composition is different from the mean composition of the fluid, can be created by electrochemical means, such as by electrolysis of a solvent, electrolysis of a dissolved species, or electrodialysis of a dissolved ionic species.
In contrast to prior art thermal flow sensors, the present invention is thermally robust in that no insulation is required to avoid thermal transients and it retains superior sensitivity as it is scaled down to the nanoliter regime and sub-millimeter dimension. Decreasing the scale of thermal flow sensors increases the heat transfer rate from the heated fluid volume to the surroundings due to a high surface-to-volume ratio and small length scale. This results in poor signal-to-noise at small flow rates (≈nL/min). In contrast, a composition pulse, such as produced here, retains its signal-to-noise ratio because mass diffusion is confined to the fluid and mass diffusivity is much smaller than thermal diffusivity. Moreover, this device is capable of operating at pressures as great as 10,000 psi, limited only by the strength of the materials of construction.
In one embodiment, the invention is directed to a device for detecting the mass flow rate of a fluid that includes:
spaced-apart electrodes disposed along the flow axis in a fluid flow channel;
means for supplying a voltage to the electrodes to produce a local compositional variation in a fluid;
means for detecting the compositional variation; and
means for determining time-of-flight of the variation.
In a second embodiment, the invention is directed to a device for detecting mass flow rate of a fluid in a capillary channel or microfluidic device that includes:
spaced-apart electrodes disposed along the flow axis in a fluid flow channel;
means for supplying a voltage to the electrodes to produce a local compositional variation in a fluid;
means for detecting the compositional variation; and
means for determining time-of-flight of the variation.
As used hereinafter, the term xe2x80x9cfluidxe2x80x9d is understood to mean a liquid that can or cannot be a solution.